Multimodal polyolefin pipe

ABSTRACT

The present invention relates to a polyethylene resin having a multimodal molecular weight distribution, said resin being further characterized in that it has a density in the range of from about 0.925 g/ccm to about 0.950 g/ccm, a melt index (I 2 ) in the range of from about 0.05 g/10 min to about 5 g/10 min, and in that it comprises at least one high molecular weight (HMW) ethylene interpolymer and at least a low molecular weight (LMW) ethylene polymer, and a composition comprising such resin. Also provided is a shaped article comprising said resin or composition, in particular a pipe.

[0001] The present invention relates to a multimodal polyethylene resin,a composition comprising such resin and to applications of such resin orcomposition, for example to make a shaped article. The resin andcomposition of the invention are particularly suitable for use in pipes.

[0002] Polyethylene compositions with a multimodal molecular weightdistribution (MWD), for example a bimodal MWD, can offer distinctadvantages compared with unimodal polyethylenes or other polyolefins.For example, bimodal polyethylenes may combine the favorable mechanicalproperties afforded by a high molecular weight polyethylene with thegood processability of a low molecular weight polyethylene. The priorart reports that such materials can advantageously be employed invarious applications, including film or pipe applications. Prior artmultimodal polyethylenes suggested for use in pipes include thematerials disclosed in the PCT applications with the publication numbersWO 97/29152, WO 00/01765, WO 00/18814, WO 01/02480 and WO 01/25328.

[0003] In view of the potentially disastrous consequences of materialfailures, acceptance of any plastic pipe for water or gas distributionis subject to product standards and performance requirements set forthin norms, for example, DIN (German Instrustrial Norm or “DeutscheIndustrie Norm”) or norms defined by ISO (International Organization forStandardization, Geneva, Switzerland). For example, state of the artpolyethylene materials sold into pipe applications, such as pressurepipes or irrigation pipes, meet the so-called PE80 or PE100 ratings (PEstands for polyethylene). Pipes manufactured from polyethylenesclassifying as PE80-type or PE100-type resins must withstand a minimumcircumferential stress, or hoop stress, of 8 MPa (PE80) or 10 MPa(PE100) at 20° C. for 50 years. PE100 resins are high densitypolyethylene (HDPE) grades typically having a density of at least about0.950 g/ccm³ or higher.

[0004] Their relatively poor Long Term Hydrostatic Strength (LTHS) athigh temperatures has been an acknowledged disadvantage of traditionalpolyethylenes which rendered these materials unsuitable for use inpiping with exposure to higher temperatures, such as domestic pipeapplications. Domestic pipe systems typically operate at pressuresbetween about 2 and about 10 bar and temperatures of up to about 70° C.with malfunction temperatures of about 95-100° C. Domestic pipes includepipes for hot and/or cold water in pressurized heating and drinkingwater networks within buildings as well as pipes for snow melt or heatrecovery systems. The performance requirements for the various classesof hot water pipes, including underfloor heating, radiator connectorsand sanitary pipes are specified, for example, in International StandardISO 10508 (first edition Oct. 15, 1995, “Thermoplastic pipes andfittings for hot and cold water systems”).

[0005] Materials which are typically used for piping with exposure tohigher temperatures include polybutylene, random copolymer polypropyleneand cross-linked polyethylene (PEX). The crosslinking of thepolyethylene is needed to obtain the desired LTHS at high temperatures.The crosslinking can be performed during extrusion, resulting in loweroutput, or in a post extrusion process. In both cases crosslinkinggenerates significantly higher costs than thermoplastic pipe extrusion.

[0006] Polyethylenes of Raised Temperature Resistance (PE-RT), asdefined in ISO-1043-1, are a class of polyethylene materials for hightemperature applications which has recently been introduced to the pipemarket. Present PE-RT resins compare unfavorably with PEX materials insome respects, for example, in that the walls of PE-RT based pipes needto be thicker than those of PEX-based pipes due to lower stress ratings.

[0007] There still is the need for new polyethylene materials whichoffer an advantageously balanced combination of thermal, mechanical andprocessing properties. In particular, there still is the need for newpolyethylene materials, which afford superior high temperatureresistance (e.g., in the range of operating temperatures from about 40°C. to about 80° C. and test temperatures of up to about 110° C.), highstress resistance, good tensile and impact performance and excellentprocessability without having to be crosslinked. It is an object of thepresent invention to meet these and other needs.

[0008] The present invention provides a polyethylene resin with amultimodal molecular weight distribution. Said multimodal polyethyleneresin is characterized in that it has a density in the range of fromabout 0.925 g/ccm to about 0.950 g/ccm and a melt index in the range offrom about 0.05 g/10 min to about 5 g/10 min.

[0009] The present invention also provides a composition comprising suchmultimodal polyethylene resin and at least one other component.

[0010] Other aspects of the invention relate to applications of suchmultimodal polyethylene resin and composition and to shaped articlesmade from such polyethylene resin or composition. One particularembodiment of the present invention relates to durable applications,such as pipes.

[0011] The term “comprising” as used herein means “including”.

[0012] The term “interpolymer” is used herein to indicate polymersprepared by the polymerization of at least two monomers. The genericterm interpolymer thus embraces the terms copolymer, usually employed torefer to polymers prepared from two different monomers, and polymersprepared from more than two different monomers, such as terpolymers.

[0013] Unless indicated to the contrary, all parts, percentages andratios are by weight. The expression “up to” when used to specify anumerical range includes any value less than or equal to the numericalvalue which follows this expression. The expression “from to” when usedto specify a numerical range includes any value equal to or higher thanthe numerical value which follows this expression. In these contexts,the word “about” is used to indicate that the specified numerical limitrepresents an approximate value which may vary by 1%, 2%, 5% orsometimes 10%.

[0014] “HMW” stands for high molecular weight, “LMW” stands for lowmolecular weight.

[0015] The abbreviation “ccm” stands for cubic centimeters.

[0016] Unless expressly specified otherwise, the term “melt index” meansthe I₂ melt index, as determined in accordance with ASTM D1238 under aload of 2.16 kg and at a temperature of 190° C.

[0017] Unless specified otherwise, the term “alpha-olefin” (α-olefin)refers to an aliphatic or cyclo-aliphatic alpha-olefin having at least4, preferably from 4 to 20 carbon atoms.

[0018] The present invention provides a polyethylene resin having amultimodal molecular weight distribution, said resin being furthercharacterized in that it has

[0019] (a) a density in the range of from about 0.925 g/ccm, preferablyof from about 0.935 g/ccm, to about 0.950 g/ccm, preferably to about0.945 g/ccm, and

[0020] (b) a melt index (I₂) in the range of from about 0.05 g/10 min,preferably of from about 0.1 g/10 min, to about 5 g/10 min, preferablyto about 1 g/10 min.

[0021] Said multimodal polyethylene resin comprises at least one highmolecular weight (HMW) ethylene interpolymer and at least a lowmolecular weight (LMW) ethylene polymer. The HMW interpolymer has asignificantly higher weight average molecular weight than the LMWpolymer. Said difference in molecular weight is reflected in distinctmelt indices. Preferred is a multimodal polyethylene resin which has atrimodal or, most preferably, a bimodal molecular weight distribution. Abimodal polyethylene resin according to the present invention consistsof one unimodal HMW ethylene interpolymer and one unimodal LMW ethylenepolymer.

[0022] The HMW component characterizing the multimodal polyethyleneresin of the invention comprises at least one or more, preferably oneHMW ethylene interpolymer. Such ethylene interpolymer is characterizedby a density in the range of from about 0.910 g/ccm, preferably of fromabout 0.915 g/ccm, to about 0.935 g/ccm, preferably to about 0.925g/ccm, and a melt index of about 1.0 g/10 min or lower, preferably ofabout 0.05 g/10 min or lower. Advantageously, the HMW ethyleneinterpolymer has a melt index of about 0.02 g/10 min or higher. The HMWethylene interpolymer contains ethylene interpolymerized with at leastone alpha-olefin, preferably an aliphatic C₄-C₂₀ alpha-olefin, and/or anon-conjugated C₆-C₁₈ diolefin, such as 1,4-hexadiene or 1,7-octadiene.Although the HMW interpolymer can be a terpolymer, the preferredinterpolymer is a copolymer of ethylene and an aliphatic alpha-olefin,more preferably such an alpha-olefin which has from four to ten carbonatoms. Particularly preferred aliphatic alpha-olefins are selected fromthe group consisting of butene, pentene, hexene, heptene and octene.Advantageously, the HMW component is present in an amount of from about30 weight percent, preferably of from about 40 percent, to about 60weight percent, preferably to about 50 percent (based on the totalamount of polymer in the multimodal polyethylene resin). Morepreferably, the HMW component is present in an amount of from about 40to about 55 percent. The molecular weight distribution as reflected bythe M_(w)/M_(n) ratio of the HMW component is relatively narrow,preferably less than about 3.5, more preferably less than about 2.4.

[0023] The LMW component characterizing the multimodal polyethyleneresin of the invention comprises at least one or more, preferably oneLMW ethylene polymer. The LMW ethylene polymer is characterized by adensity in the range of from about 0.945 g/ccm to about 0.965 g/ccm anda melt index of at least about 2.0 g/10 min or higher, preferably of atleast about 5 g/10 min, more preferably of at least about 15 g/10 min orhigher. Advantageously, the LMW component has a melt index of less than2000 g/10 min, preferably of less than 200 g/10 min. A preferred LMWethylene polymer is an ethylene interpolymer having a density in therange of from about 0.950 g/ccm to about 0.960 g/ccm and a melt index ofat least about 2 g/10 min, preferably in the range of from about 10 g/10min to about 150 g/10 min. Preferred LMW ethylene interpolymers areethylene/alpha-olefin copolymers, particularly such copolymers whereinthe aliphatic alpha-olefin comonomer has from four to ten carbon atoms.The most preferred aliphatic alpha-olefin comonomers are selected fromthe group consisting of butene, pentene, hexene, heptene and octene.Advantageously, the LMW component is present in an amount of from about40 weight percent, preferably of from about 50 percent, to about 70weight percent, preferably to about 60 percent (based on the totalamount of polymers comprised in the multimodal polyethylene resin of theinvention). More preferably, the LMW component is present in an amountof from about 45 to about 60 percent.

[0024] While the alpha-olefins incorporated into a HMW component andinto a LMW component comprised in a multimodal polyethylene resin of theinvention may be different, preferred are such multimodal polyethyleneresins, wherein the HMW and the LMW interpolymers incorporate the sametype of alpha-olefin, preferably 1-butene, 1-pentene, 1-hexene,1-heptene or 1-octene. Typically, the comonomer incorporation in the HMWethylene interpolymer is higher than in the LMW polymer.

[0025] The multimodality of a polyethylene resin according to thepresent invention can be determined according to known methods. Amultimodal molecular weight distribution (MWD) is reflected in a gelpermeation chromatography (GPC) curve exhibiting two or more componentpolymers wherein the number of component polymers corresponds to thenumber of discernible peaks, or one component polymer may exist as ahump, shoulder or tail relative to the MWD of the other componentpolymer.

[0026] For example, a bimodal MWD can be deconvoluted into twocomponents: the HMW component and the LMW component. Afterdeconvolution, the peak width at half maxima (WAHM) and the weightaverage molecular weight (M_(w)) of each component can be obtained. Thenthe degree of separation (“DOS”) between the two components can becalculated by the following equation:${DOS} = \frac{M_{w}^{H} - M_{w}^{L}}{{WAHM}^{H} + {WAHM}^{L}}$

[0027] wherein M_(w) ^(H) and M_(w) ^(L) are the respective weightaverage molecular weight of the HMW component and the LMW component; andWAHM^(H) and WAHM^(L) are the respective peak width at the half maximaof the deconvoluted molecular weight distribution curve for the HMWcomponent and the LMW component. The DOS for the bimodal resinsaccording to the invention is at least 0.01 or higher, preferably higherthan about 0.05, 0.1, 0.5, or 0.8.

[0028] WO 99/14271 also describes a suitable deconvolution technique formulticomponent polymer blend compositions.

[0029] Preferably, the HMW component and the LMW component are eachunimodal. The MWD in the GPC curves of the individual components, e.g.the HMW component and the LMW component, respectively, does notsubstantially exhibit multiple component polymers (i.e. no humps,shoulders or tails exist or are substantially discernible in the GPCcurve). Each molecular weight distribution is sufficiently narrow andtheir average molecular weights are different. The ethyleneinterpolymers suitable for use as HMW and/or LMW component include bothhomogeneously branched (homogeneous) interpolymers and heterogeneouslybranched (heterogeneous) interpolymers.

[0030] Homogeneous ethylene interpolymers for use in the presentinvention encompass ethylene-based interpolymers in which any comonomeris randomly distributed within a given interpolymer molecule and whereinall of the interpolymer molecules have substantially the sameethylene/comonomer ratio. Homogeneous ethylene interpolymers aregenerally characterized as having an essentially single melting (point)peak between −30° C. and 150° C., as determined by differential scanningcalorimetry (DSC). Typically, homogeneous ethylene interpolymers alsohave a relatively narrow molecular weight distribution (MWD) as comparedto corresponding heterogeneous ethylene interpolymers. Preferably, themolecular weight distribution defined as the ratio of weight averagemolecular weight to number average molecular weight (M_(w)/M_(n)), isless than about 3.5. Furthermore, the homogeneity of the ethyleneinterpolymers is reflected in a narrow composition distribution, whichcan be measured and expressed using known methods and parameters, suchas SCBDI (Short Chain Branch Distribution Index) or CDBI (CompositionDistribution Breadth Index). The SCBDI of a polymer is readilycalculated from data obtained from techniques known in the art, such as,for example, temperature rising elution fractionation (typicallyabbreviated as “TREF”) as described, for example, in Wild et al, Journalof Polymer Science, Poly. Phys. Ed., Vol. 20, p. 441 (1982), in U.S.Pat. No. 4,798,081 (Hazlitt et al.), or in U.S. Pat. No. 5,089,321 (Chumet al.), the disclosures of all of which are incorporated herein byreference. CDBI is defined as the weight percent of the polymermolecules having a comonomer content within 50 percent of the mediantotal molar comonomer content. The SCBDI or CDBI for the homogeneousethylene/alpha-olefin interpolymers used in the present invention istypically higher than about 50 percent.

[0031] The homogeneous ethylene interpolymers which can be used in thepresent invention fall into two categories, the linear homogeneousethylene interpolymers and the substantially linear homogeneous ethyleneinterpolymers. Both are known in the art and commercially available.

[0032] Homogeneous linear ethylene interpolymers are interpolymers whichhave a homogeneous short chain branching distribution and lackmeasurable or detectable long chain branching. Such homogeneous linearethylene interpolymers can be made using polymerization processes whichprovide a uniform branching distribution, e.g., the process described byElston in U.S. Pat. No. 3,645,992, who uses soluble vanadium catalystsystems. Other single-site catalyst systems including metallocenecatalyst systems, e.g., of the type disclosed in U.S. Pat. No. 4,937,299to Ewen et al., or U.S. Pat. No. 5,218,071 to Tsutsui et al., are alsosuitable for the preparation of homogeneous linear ethyleneinterpolymers.

[0033] The substantially linear ethylene interpolymers (SLEPs) arehomogeneous interpolymers having long chain branching, meaning that thebulk ethylene interpolymer is substituted, on average, with about 0.01long chain branches/1000 total carbons to about 3 long chainbranches/1000 total carbons (wherein “total carbons” includes bothbackbone and branch carbon atoms). Preferred polymers are substitutedwith about 0.01 long chain branches/1000 total carbons to about 1 longchain branches/1000 total carbons, more preferably from about 0.05 longchain branches/1000 total carbons to about 1 long chain branched/1000total carbons, and especially from about 0.3 long chain branches/1000total carbons to about 1 long chain branches/1000 total carbons. Thepresence of long chain branches in such ethylene interpolymers can bedetermined according to methods known in the art, such as gel permeationchromatography coupled with a low angle laser light scattering detector(GPC-LALLS) and gel permeation chromatography coupled with adifferential viscometer detector (GPC-DV).

[0034] For substantially linear ethylene polymers, the presence of longchain branching is manifest from enhanced rheological properties whichcan be quantified and expressed, for example, in terms of gas extrusionrheometry (GER) results and/or melt flow ratio (I₁₀/I₂) increases. Themelt flow ratio of the substantially linear ethylene/alpha-olefininterpolymers can be varied essentially independently of the molecularweight distribution (M_(w)/M_(n) ratio).

[0035] The substantially linear ethylene polymers are a unique class ofcompounds which has been described in numerous publications, includinge.g., U.S. Pat. Nos. 5,272,236, 5,278,272, and 5,665,800, each of whichis incorporated herein by reference. Such SLEPs are available, forexample, from The Dow Chemical Company as polymers made by the INSITE™Process and Catalyst Technology, such as AFFINITY™ polyolefin plastomers(POPs).

[0036] Preferably, SLEPs are prepared using a constrained geometrycatalyst. Such catalyst may be further described as comprising a metalcoordination complex comprising a metal of groups 3-10 or the Lanthanideseries of the Periodic Table of the Elements and a delocalized pi(π)-bonded moiety substituted with a constrain-inducing moiety, saidcomplex having a constrained geometry about the metal atom such that theangle at the metal between the centroid of the delocalized, substitutedpi-bonded moiety and the center of at least one remaining substituent isless than such angle in a similar complex containing a similar pi-bondedmoiety lacking in such constrain-inducing substituent, and providedfurther that for such complexes comprising more than one delocalized,substituted pi-bonded moiety, only one thereof for each metal atom ofthe complex is a cyclic, delocalized, substituted pi-bonded moiety.Suitable constrained geometry catalysts for manufacturing substantiallylinear ethylene polymers include, for example, the catalysts asdisclosed in U.S. Pat. Nos. 5,055,438; 5,132,380; 5,064,802; 5,470,993;5,453,410; 5,374,696; 5,532,394; 5,494,874; and 5,189,192, the teachingsof all of which are incorporated herein by reference.

[0037] The catalyst system further comprises a suitable activatingcocatalyst.

[0038] Suitable cocatalysts for use herein include polymeric oroligomeric aluminoxanes, especially methyl aluminoxane, as well asinert, compatible, noncoordinating, ion forming compounds. So-calledmodified methyl aluminoxane (MMAO) is also suitable for use as acocatalyst. Aluminoxanes, including modified methyl aluminoxanes, whenused in the polymerization, are preferably used such that the catalystresidue remaining in the (finished) polymer is preferably in the rangeof from about 0 to about 20 ppm aluminum, especially from about 0 toabout 10 ppm aluminum, and more preferably from about 0 to about 5 ppmaluminum. In order to measure the bulk polymer properties, aqueous HClis used to extract the aluminoxane from the polymer. Preferredcocatalysts, however, are inert, noncoordinating, boron compounds suchas those described in EP-A-0520732, the disclosure of which isincorporated herein by reference.

[0039] Substantially linear ethylene interpolymers are produced via acontinuous (as opposed to a batch) controlled polymerization processusing at least one reactor (e.g., as disclosed in WO 93/07187, WO93/07188, and WO 93/07189, the disclosure of each of which isincorporated herein by reference), but can also be produced usingmultiple reactors (e.g., using a multiple reactor configuration asdescribed in U.S. Pat. No. 3,914,342, the disclosure of which isincorporated herein by reference) at a polymerization temperature andpressure sufficient to produce the interpolymers having the desiredproperties. The multiple reactors can be operated in series or inparallel, with at least one constrained geometry catalyst employed in atleast one of the reactors.

[0040] Substantially linear ethylene polymers can be prepared viacontinuous solution, slurry, or gas phase polymerization in the presenceof a constrained geometry catalyst, e.g. according to the methoddisclosed in EP-A-416,815, the disclosure of which is incorporatedherein by reference. The polymerization can generally be performed inany reactor system known in the art including, but not limited to, atank reactor(s), a sphere reactor(s), a recycling loop reactor(s) orcombinations thereof and the like, any reactor or all reactors operatedpartially or completely adiabatically, nonadiabatically or a combinationof both and the like. Preferably, a continuous loop-reactor solutionpolymerization process is used to manufacture the substantially linearethylene polymer used in the present invention.

[0041] In general, the continuous polymerization required to manufacturesubstantially linear ethylene polymers may be accomplished at conditionswell known in the art for Ziegler-Natta or Kaminsky-Sinn typepolymerization reactions, that is, temperatures from 0 to 250° C. andpressures from atmospheric to 1000 atmospheres (100 MPa). Suspension,solution, slurry, gas phase or other process conditions may be employedif desired.

[0042] A support may be employed in the polymerization, but preferablythe catalysts are used in a homogeneous (i.e., soluble) manner. It will,of course, be appreciated that the active catalyst system forms in situif the catalyst and the cocatalyst components thereof are added directlyto the polymerization process and a suitable solvent or diluent,including condensed monomer, is used in said polymerization process. Itis, however, preferred to form the active catalyst in a separate step ina suitable solvent prior to adding the same to the polymerizationmixture.

[0043] Heterogeneous ethylene-based polymers encompass ethylene/α-olefininterpolymers characterized as having a linear backbone and a DSCmelting curve having a distinct melting point peak greater than 115° C.attributable to a high density fraction. Such heterogeneousinterpolymers typically have a broader molecular weight distributionthan comparable homogeneous interpolymers. Typically, heterogeneousethylene interpolymers have a CDBI of about 50% or less, indicating thatsuch interpolymers are a mixture of molecules having differing comonomercontents and differing amounts of short chain branching. Theheterogeneous ethylene polymers that can be used in the practice of thisinvention include those prepared with a coordination catalyst at hightemperature and relatively low pressure. Ethylene polymers andcopolymers prepared by the use of a (multi-site) coordination catalyst,such as a Ziegler-Natta catalyst or a Phillips catalyst, are generallyknown as linear polymers because of the substantial absence of branchchains of polymerized monomer units pendant from the backbone.

[0044] The HMW ethylene interpolymer can be a heterogeneous interpolymeror a homogeneous interpolymer, a homogeneous interpolymer beingpreferred. Particularly preferred HMW ethylene interpolymers arehomogeneous, substantially linear HMW ethylene interpolymers. The LMWethylene interpolymer can be a heterogeneous interpolymer or ahomogeneous interpolymer, a heterogeneous interpolymer being preferred.

[0045] The multimodal polyethylene resin of the invention may beprepared by any method suitable for homogeneously blendingethylene-based polymers. For example, the HMW and the LMW component canbe blended by mechanical means in the solid state, for example, inpowder or granular form, followed by melting one or both, preferablyboth of the components, using devices and equipment kown in the art.Preferably, the multimodal resin of the present invention is made byin-situ blending of the HMW component with the LMW component, e.g. usingtwo or more reactors, operated sequentially or in parallel. According toa preferred technique, the multimodal polyethylene resin of theinvention is made via the interpolymerization of ethylene and thedesired comonomer or comonomers, such as an aliphatic C₄-C₁₀alpha-olefin using a single site catalyst, e.g. a constrained geometrycatalyst in at least one reactor and a multi site catalyst in at leastone other reactor. The reactors can be operated in parallel or,preferably, sequentially. Preferably, the single site catalyst, e.g. theconstrained geometry catalyst is in the first reactor, and the multisite catalyst in the second reactor.

[0046] Most especially, a dual sequential polymerization system is used.In a preferred embodiment of the invention, the sequentialpolymerization is conducted such that fresh catalyst is separatelyinjected in each reactor. Preferably, where separate catalyst injectioninto each reactor is, no (or substantially no) live polymer or activecatalyst is carried over from the first reactor into the second reactoras the polymerization in the second reactor is accomplished only fromthe injection of a fresh catalyst and monomer (and comonomer) thereto.

[0047] In another preferred embodiment, the composition is manufacturedusing a multiple reactor system (preferably a two reactor system) inseries with fresh catalyst feed injection of a soluble catalyst systeminto the first reactor only with process adjustments being made suchthat live polymer and/or catalyst species is carried over from the firstreactor to a subsequent reactor to effect polymerization with freshmonomer and optionally comonomer.

[0048] Most preferably, whether separate injection into each reactor isused or injection into the first reactor is used, the resulting resin ischaracterized as comprising component polymers having distinct, unimodalmolecular weight distributions.

[0049] Most preferred is a multimodal polyethylene resin comprising aHMW interpolymer designated herein as preferred, more preferred orparticularly preferred and a LMW polymer designated herein as preferred,more preferred or particularly preferred, including the bimodalpolyethylene resin used to exemplify the present invention.

[0050] The present invention also provides compositions comprising themultimodal polyethylene resin of the invention and at least one otheradditional component. Preferably, such additional component is added tothe multimodal polyethylene resin of the invention. Suitable additionalcomponents include, for example, other polymers, fillers oradditives—with the proviso that these additional components do notadversely interfere with the desired advantageous properties of themultimodal polyethylene resin of the invention. Rather, the additionalcomponents are selected such as to support the advantageous propertiesof the multimodal ethylene resin of the invention and/or to support orenhance its particular suitability for a desired application. Otherpolymers comprised in the composition of the invention means polymerswhich do not qualify as a HMW interpolymer or a LMW polymer as definedherein. Advantageously, such polymers are compatible with the multimodalpolyethylene resin of the invention. Preferred additional components arenon-polymeric. Additives include processing aids, UV stabilizers,antioxidants, pigments or colorants. Most preferred are compositionscomprising a preferred, more preferred or most preferred multimodalpolyethylene resin of the invention.

[0051] The resins or compositions of the present invention can be usedto manufacture a shaped article. Such article may be a single-layer or amulti-layer article, which is obtainable by suitable known conversiontechniques applying heat, pressure or a combination thereof to obtainthe shaped article. Suitable conversion techniques include, for example,blow-molding, co-extrusion blow-molding, injection molding, injectionstretch blow molding, compression molding, extrusion, pultrusion,calendering and thermoforming. Shaped articles provided by the inventioninclude, for example, films, sheets, fibers, profiles, moldings andpipes. Most preferred is a shaped article comprising or made from apreferred, more preferred or particularly preferred resin or compositionof the present invention.

[0052] The multimodal polyethylene resins and compositions according tothe present invention are particularly suitable for durable application,especially pipes—without the need for cross-linking. Pipes comprising atleast one multimodal polyethylene resin as provided herein are anotheraspect of the present invention and include monolayer pipes as well asmultilayer pipes, including multilayer composite pipes. Typically, thepipes of the invention comprise the multimodal polyethylene resin inform of a composition (formulation) which also contains a suitablecombination of additives, e.g. an additive package designed for pipeapplications, and/or one or more fillers. Such additives and additivepackages are known in the art.

[0053] Monolayer pipes according to the present invention consist of onelayer made from a composition according to the present inventioncomprising a multimodal polyethylene resin as provided herein andsuitable additives typically used or suitable for pipe applications.Such additives include colorants and materials suitable to protect thebulk polymer from specific adverse environmental effects, e.g. oxidationduring extrusion or degradation under service conditions, such as, forexample, process stabilizers, antioxidants, pigments, metalde-activators, additives to improve chlorine resistance and UVprotectors. Preferred multilayer composite pipes include metal plasticcomposite pipes and are pipes comprising one or more, e.g., one or two,layers comprising a composition according to the present invention and abarrier layer. Such pipes include, for example, three-layer compositepipes with the general structure PE/Adhesive/Barrier orBarrier/Adhesive/PE, or five-layer pipes with the general structurePE/Adhesive/Barrier/Adhesive/PE orPolyolefin/Adhesive/Barrier/Adhesive/PE. In these structures PE standsfor polyethylene layers which can be made from the same or differentpolyethylene compositions, preferably a PE-RT comprising composition,including at least one multimodal polyethylene composition according tothe present invention. Suitable polyolefins include, for example, highdensity polyethylene, polypropylene and polybutylene, homopolymers andinterpolymers. Preferred is a multilayer composite pipe wherein at leastthe inner layer comprises a multimodal polyethylene resin according tothe present invention in a non-crosslinked form. More preferred is amultilayer composite pipe, wherein both PE layers comprise a multimodalpolyethylene resin according to the present invention. In multilayerpipes, e.g. in the three-layer and five-layer structures exemplifiedabove, the barrier layer may be an organic polymer capable of providingthe desired barrier properties, such as an ethylene-vinyl alcoholcopolymer (EVOH), or a metal, for example, aluminum or stainless steel.

[0054] The resins and compositions provided by the present invention areparticularly suitable for use in domestic and technical pipeapplications required to be operable at higher temperatures, e.g. above40° C., in particular in the range of from above 40° C. to about 80° C.Such pipe applications include, for example, hot water pipes, e.g. fordrinking and/or sanitary puposes and underfloor heating pipes. Suchpipes may be monolayer or multilayer pipes. Preferred pipes according tothe invention meet the performance requirements as defined in the normsfor hot water pipes, e.g. in ISO 10508. The multimodal polyethyleneresin according to the present invention enables pipes combining anexcellent high temperature performance, as reflected e.g. in anexcellent Long Term Hydrostatic Strength at higher temperatures (wellabove 20° C.) with good flexibility. Good flexibility facilitates e.g.pipe installation. The pipes can be produced without cross-linking,which allows improved processing economics and subsequent welding.

[0055] For plastic pipe applications, circumferential (hoop) stressperformance as set forth in ISO 9080 and ISO 1167 is an importantrequirement. The long term behaviour or lifetime of plastic pipes can bepredicted based on creep rupture data and curves which establish theallowable hoop stress (circumferential stress) which a pipe canwithstand without failure. Typically, for long term predictiveperformance testing, candidate pipe materials are subjected to variouspressures (stresses) and the lifetime at a given temperature isdetermined. For extrapolations to a lifetime of 50 years, e.g. at 20° C.to 70° C., testing is also performed at higher temperatures. Themeasured lifetime curves at each temperature typically comprise a highstress, lower lifetime ductile failure mode and a lower stress, longerlifetime brittle failure mode. A schematic representation of typicallifetime curves is found at page 412, FIG. 5, of the publication by J.Scheirs et al., TRIP 4 (12), 1996, pages 408-415. The curves can bedivided into three stages, stage I representing the ductile failurestage, stage II (knee) representing a gradual change in failure modefrom ductile to brittle, and stage III representing the brittle failurestage. Of particular interest are stages II and III, because thesestages control the lifetime of a pipe in practice. The pipes of thepresent invention show an excellent hoop stress performance particularlyat higher temperatures.

[0056] The invention is further illustrated by the following Examples,which, however, shall not be construed as a limitation of the invention.

EXAMPLES

[0057] Melt indices are expressed as I₂ (determined according to ASTMD-1238, condition E, 190° C./2.16 kg). The ratio of I₁₀ (measuredaccording to ASTM D-1238, Condition N, 190° C./10 kg) to I₂ is the meltflow ratio and designated as I₁₀/I₂.

[0058] Tensile properties, such as yield stress, yield strain, maximumtensile stress and maximum elongation, stress at break and strain atbreak are determined in accordance with ISO 527 with test specimen 5A ata test speed of 50 mm/min.

[0059] Izod impact properties are measured according to ASTM D-256.

[0060] Flexural modulus is measured according to ASTM D-790 and averagehardness D is determined according to ASTM D-2240.

[0061] The multimodal polyethylene resin used in the experiments is abimodal ethylene interpolymer having an I₂ of 0.85 g/10 min, a densityof 0.940 g/ccm and an I₁₀/I₂ of 9.8. The resin is made by in-situblending using (continuous) solution process technology and twosequentially operated reactors. The HMW ethylene interpolymer is ahomogeneous, substantially linear ethylene/octene copolymer which ismade in the primary reactor using a constrained geometry catalyst. SaidHMW interpolymer has an I₂ of 0.034 g/10 min and a density of 0.921g/ccm. The weight average molecular weight is 228,000 and the Mw/Mnratio is 2.1. The LMW ethylene polymer is a heterogeneous, linearethylene/octene copolymer having an I₂ melt index of 20 g/10 min and adensity of 0.953 g/ccm. The weight average molecular weight of the LMWpolymer is 52,100 and the Mw/Mn ratio is 3. The LMW ethylene polymer ismade in the secondary reactor using a multi-site Ziegler-Natta(coordination) catalyst. The ratio of HMW copolymer to LMW copolymer inthe bimodal polyethylene resin is 40 to 60.

[0062] The resin has the following tensile, impact and other properties(each given value represents the average of five measurements): Yieldstress [MPa]: 21 Yield strain [%]: 13 Maximum tensile stress [MPa] 36Maximum elongation [%] 760 Stress at break [MPa] 36 Strain at break [%]760 Flexural modulus [MPa] 955 Hardness D 61 Izod at 20° C. [J/m] 238Izod at −40° C. [J/m] 8

[0063] Monolithic pipes made from the above resin are subjected tohydrostatic pressure testing using the test method described in ISO 1167(1996) and water as the internal and external test medium. The pipeshave nominal dimensions of 16 mm×2 mm. The hoop stress results are givenin Table 1. Failure Temperature [° C.] Hoop Stress [MPa] Failure time[h]* Mode* 20 10.57 >3096 20 10.54 >10344 20 10.44 >10344 20 10.40 >405620 10.32 >4056 80 5.65 656 ductile 80 5.59 1245 ductile 80 5.52 >5952 805.49 >3600 80 5.45 >3600 80 5.42 >5952 80 5.35 >4056 80 5.34 >3600 805.30 >5952 80 5.25 >3600 110 2.91 >3912 110 2.89 >3912 110 2.84 >2616110 2.79 >3912 110 2.47 >11976 110 2.11 >11976

[0064] The pipes made from the bimodal polyethylene resin show anexcellent hoop stress performance, especially at high(er) temperatures.Surprisingly, no knee (stage II) reflecting a change in failure modefrom ductile to brittle is manifest, yet. Test results already go beyondthe control points for PE-RT according to DIN 16883 (1.9 MPa/8760 h at110° C.) and PEX according to ISO 10146 (2.5 MPa/8760 h at 110° C.).

What is claimed is:
 1. A polyethylene resin having a multimodalmolecular weight distribution, said resin being further characterized inthat it has (a) a density in the range of from about 0.925 g/ccm toabout 0.950 g/ccm, and (b) a melt index (I₂) in the range of from about0.05 g/10 min to about 5 g/10 min, and (c) comprises at least one highmolecular weight (HMW) ethylene interpolymer and at least a lowmolecular weight (LMW) ethylene polymer, wherein (d) the HMW componentcomprises at least one or more ethylene interpolymers having a densityin the range of from about 0.910 g/ccm to about 0.935 g/ccm, and a meltindex of about 1.0 g/10 min or lower, and (e) the LMW componentcomprises at least one or more ethylene polymers having a density in therange of from about 0.945 g/ccm to about 0.965 g/ccm and a melt index ofat least about 2.0 g/10 min or higher.
 2. The polyethylene resinaccording to claim 1 which has a bimdoal molecular weight distributionand consists of one unimodal HMW ethylene interpolymer and one unimodalLMW ethylene polymer.
 3. The polyethylene resin according to any ofclaims 1 to 2, wherein the LMW ethylene polymer is an ethyleneinterpolymer and has a density of 0.960 g/ccm or lower.
 4. Thepolyethylene resin according to any of claims 1 to 3, wherein the HMWand/or the LMW ethylene interpolymer is a homogeneous, substantiallylinear interpolymer.
 5. A composition comprising the multimodalpolyethylene resin according to any of claims 1 to 4 and at least oneother additional component.
 6. The composition according to claim 5wherein the at least one other additional component is selected from thegroup consisting of fillers and additives.
 7. A shaped articlecomprising the multimodal polyethylene resin according to any of claims1 to
 5. 8. The shaped article according to claim 7 which is a pipe. 9.The pipe according to claim 8 which is operable at temperatures of above40° C.
 10. The pipe according to claim 8 which is a hot water pipe,preferably such pipe meeting the performance requirements defined in theISO 10508 standard.